In view of grasping movement of an object, detection of acceleration has important meaning. For this reason, various acceleration sensors have been conventionally proposed. Particularly, in recent years, the spotlight of attention upon multi-dimensional acceleration sensors capable of detecting two-dimensional or three-dimensional acceleration every respective directional components has been focused. For example, in the International Publication No. WO88/08522 based on the Patent Cooperation Treaty, a three-dimensional acceleration sensor using piezo resistance elements is disclosed. In this sensor, plural piezo resistance elements are formed at specific positions on a semiconductor substrate, thereby making it possible to respectively independently detect acceleration components in respective coordinate axes directions in the XYZ three-dimensional coordinate system. Moreover, in the International Publication No. WO91/10118 or the International Publication No. WO92/17759, a three-dimensional acceleration sensor using electrostatic capacitance elements is disclosed. In the International Publication No. WO93/02342, a three-dimensional acceleration sensor using piezoelectric elements is disclosed. In these sensors, plural electrodes are formed at specific positions, thereby making it possible to respectively independently detect acceleration components in respective coordinate axes directions in the XYZ three-dimensional coordinate system in a manner similar to the above.
In such three-dimensional acceleration sensors, since all of respective coordinate axes direction components of applied acceleration can be respectively independently detected by a single sensor, it is possible to specify an acceleration to be detected as a vector quantity within the three-dimensional space. Accordingly, such three-dimensional acceleration sensors can be widely utilized in use for precisely detecting acceleration exerted on an object moving within the three-dimensional space, e.g., a vehicle being traveled, or an air-plane being flown, etc. in a manner to include its direction. In future, it is expected that its utilization value will be increased.
On the other hand, the acceleration sensor can be utilized also as a seismometer or an impact detector. For example, in a control system for control valves of the city gas or a control system for control an elevator, an acceleration sensor functioning as a seismometer is included. In the case where an acceleration based on vibration of the earthquake exceeds a predetermined threshold value, the control system stops supply of gas, or stops an operation of the elevator. Moreover, in automotive vehicles with an air bag which begins being popularized rapidly in recent years, an acceleration sensor functioning as an impact detector is mounted, wherein there is employed a mechanism to momentarily swell the air bag to protect a driver in the case where an acceleration based on impact exceeds a predetermined threshold value. However, it is to be noted that the acceleration sensors used in seismometers or impact detectors are mainly mechanical type sensors which detect an acceleration by means of mechanical operation. For example, a mechanical sensor recognizes whether or not a steel ball is flown out from a bowl-shaped vessel, and judges whether or not an acceleration more than the threshold value is exerted.
As described above, the acceleration sensors utilized as seismometers or impact detectors are mainly mechanical type sensors for the present. However, such mechanical acceleration sensors have drawbacks that the detection accuracy or the reliability is low, and it is difficult to electrically take out detection results. On the other hand, three-dimensional acceleration sensors using piezo resistance elements, capacitance elements or piezoelectric elements have a high detection accuracy and reliability, and can electrically take out detection results. However, for the purpose of the seismometer or the impact detector, such three-dimensional acceleration sensors are not necessarily required. On the contrary, there are even instances where the conventional three-dimensional acceleration sensor is difficult to use.
For example, for the purpose of measuring an intensity of the earthquake, it is sufficient to provide a function to respectively independently detect a magnitude of "transverse vibration (vibration in the horizontal direction)" and a magnitude of "longitudinal vibration (vibration in the vertical direction)". In this case, it is desirable to have an ability of directly detecting the magnitude of "transverse vibration" and the magnitude of "longitudinal vibration". Generally, it is known that the "transverse vibration" in the earthquake is a vibration resulting from a wave called "S-wave", and the "longitudinal vibration" is a vibration resulting from a wave called "P-wave". As long as it is possible to respectively independently detect the magnitude of the S-wave and the magnitude of the P-wave, such a detection mechanism can sufficiently function as a seismometer. Namely, when an XYZ three-dimensional coordinate system respectively having an XY-plane on the horizontal surface and a Z-axis in the vertical direction is defined at a certain measurement point, if a magnitude of vibration in the direction along the XY-plane (transverse vibration) and a magnitude of vibration in the direction along the Z-axis (longitudinal vibration) can be measured, such a measurement mechanism can sufficiently function as a seismometer
It is a matter of course that even if a conventionally proposed three-dimensional acceleration sensor is employed, such a measurement can be made. If the conventional three-dimensional acceleration sensor is used as a seismometer, a precise detection can be made in which even a particular direction of vibration is specified such as "transverse vibration directed to north-northeast", etc. However, for the purpose of carrying out supply control of the city gas or operation control of an elevator, it is unnecessary to specify as far as azimuth (direction) of the transverse vibration. Actually, when a magnitude of transverse vibration exceeds a predetermined threshold value, irrespective of "transverse vibration directed to north-northeast" or "transverse vibration directed to southeast", it is necessary to stop the supply of the city gas or the operation of the elevator. As long as even a magnitude of the transverse vibration can be detected, it is possible to sufficiently perform the function as an seismometer. Moreover, in the conventional three-dimensional acceleration sensor, since X-axis direction component .alpha.x, Y-axis direction component .alpha.y and Z-axis direction component .alpha.z are respectively independently detected with respect to an acceleration in the XYZ three-dimensional coordinate system, an operation (calculation) to obtain a sum of .alpha.x.sup.2 +.alpha.y.sup.2 and to obtain a square root of this sum is required in order to determine a magnitude of the transverse vibration along the XY plane.
As stated above, the conventional three-dimensional acceleration sensors can be used as a seismometer. However, since the structure thereof is complicated and a particular operation circuit is required to function as a seismometer, there results the problem that the cost is increased as a whole. Particularly, when attention is drawn to utilization to the supply control of city gas or the operation control of elevator, there is the necessity of respectively installing such an acceleration sensor within every gas meter of respective homes or every control unit of respective elevators. Therefore, realization of low cost acceleration sensors having simple structure is expected.
Such circumstances are the same also in an acceleration sensor used as an impact detector for operating an air bag of the automotive vehicles. If the traveling surface of the automotive vehicles is assumed to be XY plane, an impact produced by collision of the automotive vehicle is the impact mainly including an acceleration component along the XY plane. Accordingly, an acceleration component along the Z-axis can be neglected. In addition, irrespective of whether the corresponding collision is a frontal (head-on) collision or a side collision, the fact that the impact which allows a driver to suffer from risk is applied is the same. Namely, even if a collision in any direction takes place, there is the necessity of swelling the air bag to protect the driver. Accordingly, when a magnitude of acceleration components in the directions along the XY plane can be detected, such an acceleration detection system sufficiently satisfies the purpose. Namely, it is unnecessary to precisely detect as far as the direction.
With the above in view, an object of this invention is to provide an acceleration sensor suitable for detecting, as an electric signal, a magnitude of acceleration directed toward the directions included within a predetermined plane.